A capsule of this kind has been described, for example, in U.S. Pat. No. 4 861 520; it is filled with radioactive pellets, closed with a stopper, and the stopper is then joined to the capsule by electron beam or laser beam welding. The cable is then welded to the stopper, also using a laser beam or electron beam. Since the diameter of such radiation sources, which are used in medical technology in reloadable apparatus, is from 1 to 3 mm, it is difficult to form the welded joints so that they meet the standards in respect of resistance to breakage. For this purpose the capsule, the stopper and the cable must be brought together in holders and rotated through 360.degree. during the welding process so that the weld bead extends right round the circumference of the capsule to give an absolutely leak-proof connection. Moreover electron beam welding must be performed in vacuum, and is therefore a lengthy and expensive process.